The human spine is composed of many vertebra stacked one upon the other, with an intervertebral disc between each pair of adjacent vertebra. The discs act as cartilaginous cushions and shock absorbers. The spinal cord runs in a bony canal formed by successive openings in these bones. The spinal nerves exit the spinal cord between pairs of vertebrae and supply nerves and nerve signals to and from other body structures.
The vertebral disc is a complex joint both anatomically and functionally. It is composed of three component structures: the nucleus pulposus; the annulus fibrosis, and the vertebral end plates. The biomedical composition and anatomical arrangements within these component structures are related to the biomechanical function of the disc.
The nucleus pulposus, occupying about 24% to 40% of the total disc cross-sectional area, usually contains approximately 70% to 90% water by weight.
The annulus fibrosis is a concentrically laminated structure which contains highly aligned collagen fibers and fibril cartilage embedded in an amorphous round substance. The annular layers are oriented at approximately +/−60° to the longitudinal axis of the spine. The annulus fibrosis usually contains approximately eight to ten layers and is mechanically the main stabilizing structure which resists torsional and bending forces applied to the disc.
The two vertebral end plates separate the disc from the adjacent vertebral bodies, and are composed of hyaline cartilage.
Spinal discs may be damaged or displaced due to trauma or disease. In either case, the nucleus pulposus may herniate and protrude into the vertebral canal or intervertebral foramen. This condition is known as a herniated or “slipped” disc. This may in turn press upon the spinal nerve that exits the vertebral canal through the partially obstructed foramen, causing pain or paralysis in the area of its distribution. The most frequent site of occurrence of a herniated disc is in the lower lumbar region. To alleviate this condition, two procedures are common.
First, it may be necessary to remove the involved disc surgically and fuse the two adjacent vertebrae together. Spinal fusion is a good method of eliminating symptoms, but at the expense of total loss of motion of the fused vertebral joint, as well as increased stress in the adjacent segments. In many long term patients of fused spinal segments, a detrimental phenomena has been observed whereby discs adjacent to the fused spinal segment will have increased motion and stress due to the increased stiffness of the fused segment. This is sometimes referred to “cascading spine syndrome,” where previously normal motion segments above or below a fused segment exhibit spondylolisthesis, or degenerative disc disease due to increased loading. A second method for alleviating disc problems is insertion of an intervertebral disc replacement. The object of an intervertebral disc replacement is to provide a prosthetic disc that combines both stability to support high loads of patient vertebrae and flexibility to provide the patient with sufficient mobility and proper spinal column load distribution. In attempting to satisfy these competing design requirements, basically four types of intervertebral discs have been developed; elastomer disc, ball and socket disc, mechanical spring disc, and hybrid discs.
No matter which of the artificial intervertebral disc replacements are used, all lack memory storage and processing capabilities that would assist a doctor in diagnosing a patient's spinal condition and the performance of the artificial disc once implanted. Once the artificial disc is inserted into the patient's spine, the doctor is required to rely upon verbal feedback and imaging techniques from the patient in order to diagnose any problems that may be occurring with respect to the artificial disc. This type of feedback is limited due to the lack of ability of the patient to properly describe sensations or feelings which may be coming from their back, and may further be limited by a lack of candor on the part of the patient who may fail to tell the doctor about activities the patient has engaged in that are not appropriate for the artificial disc. Thus, there is a need for a type of artificial intervertebral disc that provides processing and memory storage capabilities that would assist in the collection of data relating to the intervertebral disc that could be used either output in real time to a doctor diagnosing a patient, or alternatively, could be downloaded from the memory to provide information as to the patient's load history.